Santos made the announcement Wednesday night. He was first elected president in 2010.
A recent poll puts Santos ahead of four other possible candidates for the May election, but with a relatively low support level.
The poll by Invamer-Gallup says 27 percent of people surveyed said they would vote for Santos, with former Finance Minister Oscar Ivan Zuluaga in second place at 15 percent. The poll questioned 1,200 people between Nov. 1 and 6, and had a margin for error of three percentage points.
Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.